test against 2.0
use https in gemfile
register embedded engine for asciidoc

- asciidoc content handled by Asciidoctor by way of Tilt 1.3.4
- document embedded asciidoc engine to README.md
- add asciidoctor as development dependency
Merge pull request #357 from mojavelinux/asciidoc

register embedded engine for asciidoc
readme cleanup

[ci skip]
changes

[ci skip]
Merge remote-tracking branch 'slim/master' into README_ja

Conflicts:
	README.md